[PATCH 14/14] ARM: elf: add new hwcap for identifying atomic ldrd/strd instructions

[Will Deacon]

On Wed, May 22, 2013 at 09:47:44AM +0100, Catalin Marinas wrote:
> On Tue, May 21, 2013 at 07:48:35PM +0100, Rob Herring wrote:
> > On Mon, May 20, 2013 at 9:18 AM, Catalin Marinas
> > <catalin.marinas at arm.com> wrote:
> > > On Fri, May 17, 2013 at 06:07:53PM +0100, Will Deacon wrote:
> > >> CPUs implementing LPAE have atomic ldrd/strd instructions, meaning that
> > >> userspace software can avoid having to use the exclusive variants of
> > >> these instructions if they wish.
> > >>
> > >> This patch advertises the atomicity of these instructions via the
> > >> hwcaps, so userspace can detect this CPU feature.
> > >>
> > >> Reported-by: Vladimir Danushevsky <vladimir.danushevsky at oracle.com>
> > >> Signed-off-by: Will Deacon <will.deacon at arm.com>
> > > ...
> > >> +
> > >> +     /* LPAE implies atomic ldrd/strd instructions */
> > >> +     vmsa = (read_cpuid_ext(CPUID_EXT_MMFR0) & 0xf) >> 0;
> > >> +     if (vmsa >= 5)
> > >> +             elf_hwcap |= HWCAP_LPAE;
> > >
> > > As I mentioned in the past, I don't agree with exposing the "LPAE"
> > > feature to user-space, it's not a feature that user space should care
> > > about. An atomic double hwcap is better and you can even make this per
> > > CPU via __v7_proc.
> > 
> > How does userspace know whether to install a non-LPAE or LPAE kernel
> > in a generic way?
> 
> This is a valid reason to expose LPAE to user, though elf_hwcap sounds a
> bit strange.

In lieu of anything else, do you mind if I continue with the patch as it
stands?

Will